In electric discharge machining, a liquid dielectric, such as oil or water, is generally used as an EDM fluid. It is also known to use EDM fluid which is produced by admixing conductive or semiconductive powder into the liquid dielectric. The liquid dielectric mixed with the conductive powder is used for machining a workpiece by electrical discharges through the mixed liquid and conductive powder, and there have been proposals thereon as follows.
Japanese Patent Publication No. 52-26357, published on Jul. 13, 1977, discloses a method of adding conductive powder to the liquid dielectric supplied into the gap between a tool electrode and a workpiece and of controlling the amount of the powder according to a desired machining over-size. The conductive powders to be added are produced by mixing metal powder or graphite powder having a particle size of 5 to 10 micrometers with machined chips such as graphite and carbon black of 10 to 100 angstroms. The mixed conductive powders are admixed to the liquid dielectric to a density from 0.05 to 15 g/liter.
Japanese Patent Publication No. 55-27810, published on Jul. 23, 1980, discloses a method of admixing metal powders, susceptible of vaporization (i.e., with high vapor pressure), with the liquid dielectric to be supplied to the machining gap. The metal powders are easily vaporized at a high temperature of electric discharges and the resultant metal vapor becomes dispersed in the liquid dielectric increasing conductivity thereof to thereby render it easier to induce electric discharges when applying voltage pulses across the machining gap. Metal powders of cadmium, indium, zinc, magnesium, tin, lead, and bismuth are used.
Japanese Laid-Open Patent Publication No. 55-500783, published on Oct. 16, 1980, discloses that a liquid dielectric mixed with conductive powder such as metal powder or graphite of up to 5 micrometers particle size at 0.001 to 1% by volume increases removal rate with a reduction of electrode wear rate and an improvement in the machined surface.
Japanese Patent Publication No. 54-20718, published on Jul. 25, 1979, discloses an electric discharge machining method in which a paste produced by kneading a mixture of a powder mainly comprising conductive powder and the liquid dielectric, is disposed in the desired machining gap. As conductive powders, graphite, brass, tungsten, silver tungsten, silver, copper, copper tungsten, are used. This publication also discloses that a paste may be produced by kneading a mixture of conductive powder and a magnetic powder of barium ferrite and high carbon steel with a small amount of liquid dielectric.
Japanese Laid-Open Patent Publication No. 2-83119, published on Mar. 23, 1990, discloses a method of forming a surface layer on a workpiece by means of electrical discharges in which the material for forming the surface layer is disposed between a tool electrode and the workpiece in powder form. The EDM fluid includes 20 g/liter silicon having an average particle size of about 20 to 40 micrometers mixed with the liquid dielectric. Further, as the powder materials, tungsten carbide and semiconductor such as zirconium boride, and others are presented.